This invention relates to optical fiber transmission systems, and in particular to an apparatus and method for measuring and controlling polarization mode dispersion in such systems.
The measurement of optical signal parameters has taken on increased importance as digital communications migrates from copper and radio networks to optical fiber transmission systems. With the advent of wavelength division multiplexed optical fiber communication systems (WDM systems and dense WDM systems), fiber optic bandwidth is being used for the simultaneous transmission of many data channels at different optical frequencies.
Efficient fiber data transmission requires increasingly precise and high tolerance measurement of various optical parameters. One such parameter is polarization mode dispersion (PMD). Imperfections in fibers, particularly the long haul fibers, cause differing delays the orthogonal polarization components of an optical signal. These differing delays produce polarization mode dispersion (PMD) which can be distort the shape of transmitted signal pulses. PMD is of increasing concern at high bit rates, particularly at 40 Gb/s and above.
In accordance with the invention, an optical fiber transmission system is provided with a new method and apparatus to measure PMD and then to use the measurements to control adaptive optical filters that minimize system signal degradation. The measurements provide accurate information to compensate for distortions and thereby permit higher transmission rates. The new technique can obtain phase information without optical heterodyning and without bandlimiting.
In a first embodiment, an optical signal transmitter, such as a laser/modulator pair, is driven by a data signal combined with a test signal, in the form of a comb of tones with known relative magnitude and phase relationships. Tunable all pass filters recover phase and magnitude information to control an adaptive filter for precise compensation.
In a second embodiment, no changes are made to the transmitted signal. The relative delay between orthogonal polarizations of a modulated signal can be obtained by using a polarization beam splitter (PBS), flipping (rotating by 90xc2x0) the polarization of one output, sweeping a narrowband optical filter (NBF) across the signal spectrum and detecting the interference between polarizations. Additional detectors can obtain the magnitude versus frequency for the orthogonal components, x and y.